Rainbows On Titan

Titan might be the only other place in our solar system that has rainbows.

Rainbows On Titan!

by Dr Tony Phillips

When the European Space Agency's Huygens probe visited Saturn's moon Titan last month, the probe parachuted through humid clouds. It photographed river channels and beaches and things that look like islands.
Finally, descending through swirling fog, Huygens landed in mud. To make a long story short, Titan is wet.
Christian Huygens wouldn't have been a bit surprised. In 1698, three hundred years before the Huygens probe left Earth, the Dutch astronomer wrote these words:
"Since 'tis certain that Earth and Jupiter have their Water and Clouds, there is no reason why the other Planets should be without them. I can't say that they are exactly of the same nature with our Water; but that they should be liquid their use requires, as their beauty does that they be clear.
"This Water of ours, in Jupiter or Saturn, would be frozen up instantly by reason of the vast distance of the Sun. Every Planet therefore must have its own Waters of such a temper not liable to Frost."
Huygens discovered Titan in 1655, which is why the probe is named after him. In those days, Titan was just a pinprick of light in a telescope. Huygens could not see Titan's clouds, pregnant with rain, or Titan's hillsides, sculpted by rushing liquids, but he had a fine imagination.
Titan's "water" is liquid methane, CH4, better known on Earth as natural gas. Regular Earth-water, H2O, would be frozen solid on Titan where the surface temperature is 290o F below zero. Methane, on the other hand, is a flowing liquid, of "a temper not liable to Frost."
Jonathan Lunine, a professor at the University of Arizona, is a member of the Huygens mission science team. He and his colleagues believe that Huygens landed in the Titan-equivalent of Arizona, a mostly-dry area with brief but intense wet seasons.
"The river channels near the Huygens probe look empty now," says Lunine, but liquids have been there recently, he believes. Little rocks strewn around the landing site are compelling: they're smooth and round like river rocks on Earth, and "they sit in little depressions dug, apparently, by rushing fluids."
The source of all this wetness might be rain. Titan's atmosphere is "humid," meaning rich in methane. No one knows how often it rains, "but when it does," says Lunine, "the amount of vapor in the atmosphere is many times that in Earth's atmosphere, so you could get very intense showers."
And maybe rainbows, too. "The ingredients you need for a rainbow are sunlight and raindrops. Titan has both," says atmospheric optics expert Les Cowley.
On Earth, rainbows form when sunlight bounces in and out of transparent water droplets. Each droplet acts like a prism, spreading light into the familiar spectrum of colors. On Titan, rainbows would form when sunlight bounces in and out of methane droplets, which, like water droplets, are transparent.
"Their beauty [requires] that they be clear...."
"A methane rainbow would be larger than a water rainbow," notes Cowley, "with a primary radius of at least 49o for methane vs 42.5o for water. This is because the index of refraction of liquid methane (1.29) differs from that of water (1.33)."
The order of colors, however, would be the same: blue on the inside and red on the outside, with an overall hint of orange caused by Titan's orange sky.
One problem: Rainbows need direct sunlight, but Titan's skies are very hazy. "Visible rainbows on Titan might be rare," says Cowley. On the other hand, infrared rainbows might be common.
Atmospheric scientist Bob West of NASA's Jet Propulsion Laboratory explains: "Titan's atmosphere is mostly clear at infrared wavelengths. That's why the Cassini spacecraft uses an infrared camera to photograph Titan."
Infrared sunbeams would have little trouble penetrating the murky air and making rainbows. The best way to see them: infrared "night vision" goggles.
All this talk of rain and rainbows and mud makes liquid methane sound a lot like ordinary water. It's not. Consider the following:
The density of liquid methane is only about half the density of water. This is something, say, a boat builder on Titan would need to take into account. Boats float when they're less dense than the liquid beneath them.
A Titan-boat would need to be extra lightweight to float in a liquid methane sea. (It's not as crazy as it sounds. Future explorers will want to visit Titan and boats could be a good way to get around.)
Liquid methane also has low viscosity (or "gooiness") and low surface tension. See the table below. Surface tension is what gives water its rubbery skin and, on Earth, lets water bugs skitter across ponds.
A water bug on Titan would promptly sink into a pond of flimsy methane. On the bright side, Titan's low gravity, only one-seventh Earth gravity, might allow the creature climb back out again.
Back to boats: Propellers turning in methane would need to be extra-wide to "grab" enough of the thin fluid for propulsion. They'd also have to be made of special materials resistant to cracking at cryogenic temperatures.
And watch out for those waves! European scientists John Zarnecki and Nadeem Ghafoor have calculated what methane waves on Titan might be like: seven times taller than typical Earth-waves (mainly because of Titan's low gravity) and three times slower, "giving surfers a wild ride," says Ghafoor.
Last but not least, liquid methane is flammable. Titan doesn't catch fire because the atmosphere contains so little oxygen - a key ingredient for combustion. If explorers visit Titan one day they'll have to be careful with their oxygen tanks and resist the urge to douse fires with "water."
Infrared rainbows, towering waves, seas beckoning to sailors. Huygens saw none of these things before it plopped down in the mud. Do they really exist?
"...there is no reason why the other Planets should be without them."


http://www.spacedaily.com/news/saturn-titan-05p.html

Image above: A sun-like star grows into its red giant phase, increasing in size and luminosity. Energy in the form of heat can now reach a once-frozen and dead moon. The icy surface quickly melts into liquid water, filling in old craters with warmer seas. The stage is now set for the possible formation of new life.

New Frontier Opens In The Search For Life On Other Planets

Scientists recently discovered a new frontier in the race to find life outside our solar system. Dying red giant stars may bring icy planets back from the dead. Once-frozen planets and moons may provide a new breeding ground for life as their stars enter the last, and brightest, phase of their lives. Previous ideas about the search for extra-solar life had excluded these regions.

An international team of astronomers estimates that the emergence of new life on a planet is possible within the red giant phase. "Our result indicates that searches for life-giving worlds outside our solar system should include planets around old stars," said Dr. Bruno Lopez of the Observatoire de la Cote d'Azur, Nice, France. Lopez and his colleagues estimate that more than 150 red giant stars are close enough - within 100 light years - for upcoming or proposed missions to search for the signatures of life on distant worlds. A light year is the distance light travels in one year, almost six trillion miles!

Location, Location, LocationOne of the secrets of Earth's success in producing life is its location within the sphere of the Sun's habitable zone. This sphere intersects the plane of the solar system to create a special donut-shaped boundary that outlines where water can exist as a liquid in our solar system, a necessity for the development of life. Get too far from the Sun - and it's a lonely icebox. Too close - and the water evaporates into space, never to return again.

While the Earth currently sits well within this donut of life, our Sun is evolving and will one day grow to be a red giant star. Its habitable zone will expand with it, changing the locales where liquid water can splash and life may one day thrive.

Lying just inside the outer limit of our Sun's habitable zone, Mars remains a frozen world today because of its thin atmosphere. However, when the Sun becomes a red giant a few billion years from now, Mars may become the happening place to be. "Mars will be in the habitable zone for a couple billion years, so Martian life may get a second chance," said Dr. William Danchi of NASA's Goddard Space Flight Center, Greenbelt, Md.

In 2003, researchers monitored the amount of ice on Mars during its winter and spring seasons. In some regions, the water-ice content was more than 90% by volume. Scientists suspect that this water used to fill the planet's now-dry lakes and seas. One day in the distant future, the frozen water on Mars may fill these dry basins again and bring forth new life in our solar system.

Red Giants Redefine the Search for Extra-Terrestrial LifeThe same holds true for planets and moons as they orbit their own red giant suns. Billions of years ago, these stars were similar to our Sun. Imagine the events as they unfolded: A Sun-like star explodes into its red giant phase, growing tremendously in size and brightness. Warm rays from the star reach out to a once-frozen and dead moon. The solitary satellite's icy top layer quickly melts into liquid water, which creeps across the surface and fills old dusty craters with warmer seas. The stage is set for the birth of new life in the moon's now-vibrant oceans.

Currently, there are at least 150 red giant stars within 100 light years of Earth and many of them may have orbiting planets capable of supporting life. A new frontier has opened for planet-hunters around the world.

One such endeavor, NASA's Kepler mission, hopes to discover smaller Earth-like planets outside our solar system. Looking for tiny dips in the brightness of a star when a planet crosses in front of it, researchers will observe about 100,000 stars in one small patch of sky for four years. Kepler is set for launch in 2007.

Image above: Our planet lies within the Sun's habitable zone. This donut-shaped boundary outlines where water can exist as a liquid in our solar system, a necessity for the development of life. As the Sun develops into old age, its habitable zone will expand with it, changing the locales where liquid water can splash and life may one day thrive.

Although Mars is now very dry, this sequence illustrates what Mars would look like today if it still had the massive amount of water researchers believe was present at its formation. An elevation map was created from a topographic survey by NASA's Mars Global Surveyor (MGS) spacecraft, and a computer artist filled in the ancient Martian ocean.

In 2003, researchers monitored the amount of water-ice on Mars, which is more than 90% in some areas. This sequence begins with a "true-color" mosaic of Mars from the Viking orbiter and ends with a visualization of water ice concentration, shown in blue, during the Martian summer months.

Researchers estimate that the time required for the emergence of new life fits within the Red Giant phase, adding to the number of stars that may have Earth-like planets orbiting them. The Kepler Mission will look for tiny dips in the brightness of a star when a planet crosses in front of it, known as a transit.

http://www.nasa.gov/vision/universe/starsgalaxies/frozenworlds.html

Weighing a Black Hole

Scientists have weighed a black hole by observing strong X-ray outbursts from it. The timing and regularity of the bursts – seen by NASA’s Chandra X-ray Observatory – imply an object 10,000 times more massive than our Sun. This might sound like a bundle, but in the boxing ring of black holes, it qualifies only as middleweight.

Astronomers have previously observed stellar-mass black holes – with about 10 solar masses – and supermassive black holes – with a million or more solar masses. The recent measurements of a black hole in M74, a galaxy 32 million light years away, are the best evidence so far for an intermediate-mass black hole.

"It is important to verify the existence of intermediate-mass black holes, because they would bridge the gap between stellar-mass black holes and supermassive black holes in the centers of galaxies," said Jifeng Liu of the University of Michigan in Ann Arbor.

Liu and his colleagues found that the M74 source varied in its X-ray brightness every two hours, providing an important clue to the black holes' mass.

Some scientists had speculated that ultraluminous X-ray sources (ULXs), of which the M74 object is one, are stellar-mass black holes that look brighter because they are beaming X-rays directly at Earth. However, the variation pattern observed by Liu’s team seems to require a bigger black hole.

If the object is indeed one of the elusive intermediate-mass black holes, the next question is how did it form. One of the leading theories is that hundreds of stellar-mass black holes (which form out of the deaths of massive stars) merge together at the center of a dense star cluster.

Another possibility is that the intermediate object was the central black hole of a small galaxy that is being eaten by the larger M74 galaxy.

The results appeared in the March 1 issue of the Astrophysical Journal Letters.

CNN.com - Nearby evidence for dark energy - Mar 22, 2005

A cross-section of the universe shows galaxies as bright dots along filaments of matter, with a sea of dark energy filling in between the galactic islands.
CNN.com - Nearby evidence for dark energy - Mar 22, 2005

(SPACE.com) -- New research suggests evidence of dark energy in our cosmic backyard, but theorists are still divided on explanations for the ever-increasing speed with which the universe is expanding.
Until now, evidence for dark energy, a mysterious antigravity force apparently pushing galaxies outward at an accelerating pace, has only been found in the farthest reaches of the universe.
But an international team of researchers has used computer models supported by observations from the Hubble Space Telescope to find hints of dark energy closer by.
In the 1920's, astronomer Edwin Hubble showed that the universe was expanding and that galaxies were moving away from each other with speeds proportional to their distance. He measured distances of other galaxies from the Earth by their brightness as an indication. If the light of a galaxy became fainter over time, it meant that it was moving further away.
From this, scientists observed that the universe had been expanding since the Big Bang, the theoretical instant that the universe began with a giant rush of energy. It was assumed that this spreading out of our universe was occurring at a constant rate and that the tug of gravity among galaxies could possibly slow universe's expansion over time.
But in the 1990's, scientists observed that exploding stars in very distant galaxies were dimmer than expected. This meant that those galaxies were farther than calculated. Not only is the universe expanding, but it is doing so at an ever-increasing pace, the data revealed.
At a loss to explain this discovery, cosmologists concluded that an unexplained force, dubbed dark energy, was causing the acceleration.
Dark energy is thought to work on grand scales. Locally, gravity holds an individual galaxy together and even binds clusters of galaxies.
In 1997, Fabio Governato, of University of Washington and Italy's National Institute for Astrophysics, designed a computer model to simulate universal expansion from the Big Bang to the present. The simulated universes -- several scenarios -- were then compared with data of our actual one.
The model, however, could not duplicate the patterns of expansion of the Local Group of galaxies, a huddle of more than 30 galaxies that includes our Milky Way. The simulation results predicted expansion speeds three to seven times faster than those detected by astronomers.
It was only when Governato and his research team incorporated dark energy into the initial conditions of their simulation model and to the equation that regulates the expansion of the universe, that they were able to closely match the observed expansion speeds of galaxies nearby.
"Adding dark energy changes the equation that describes the speed at which the universe expands with time and the growth of density perturbations," Governato told SPACE.com.
"The observed motion was small, and we could not duplicate it without the presence of dark energy," Governato said. "When we added the dark energy, we got a perfect match."
The scenario was presented last week and will be published in the Monthly Notices of the Royal Astronomical Society.
The simulated universe is a sea of dark energy acting on billions of floating galactic islands, including the Milky Way and the other members of the local group, Governato and his colleagues say.
"We studied the properties of galaxies close to the Milky Way instead of looking billions of light years away," Governato said. "It's like traveling from Seattle to Portland, Oregon, rather than from Seattle to New York, to measure the Earth's curvature."
But not everyone's convinced that the answer lies in the dark.
"The hypothesis of dark energy is extremely fascinating," says physicist Antonio Riotto of Istituto Nazionale di Fisica Nucleare of Padova, Italy. But no theoretical model has yet satisfactorily explained its presence.
Riotto and a team of theoretical physicists in the US and Canada say there's no need to add dark energy as a new ingredient to explain the increasing speed of universal expansion.
"Our solution to the paradox posed by the accelerating universe relies on the so-called inflationary theory, born in 1981," Riotto proposed last week. According to this theory, the universe went through a period of exponential expansion in the moment immediately following the Big Bang. During this expansion, tiny ripples in space-time, much like those produced by a rock thrown in a pool, were produced.
With a universe that is infinite, these ripples continue to stretch and grow with expansion and over time are causing cosmic acceleration.
New York University physicist Georgi Dvali sees the answer in yet another theoretical phenomenon: leaking gravity.
His hypothesis relies on string theory, which states that there are extra, hidden dimensions beyond the three directions and time. In such a case, gravitons, hypothetical elementary particles transmitting gravitational forces, may escape to other dimensions.
This, Dvali explained earlier this year, would cause "leaks" in gravity over cosmic proportions, reducing gravitational pull at larger distances, altering the space-time continuum and effectively speeding up universal expansion.
The answer appears as far away as the stars for now as theorists and observational astronomers grapple with one of the toughest questions on their agenda.

Huge Space Clouds May Have Caused Mass Extinctions

Giant space clouds of gas may have changed the climate or atmosphere on Earth and fueled mass extinctions millions of years ago, scientists said Thursday.
In one scenario, the solar system passed through a dense cloud of interstellar material, causing Earth to ice over. In the other, the solar system passed through less dense clouds that destroyed the planet's protective ozone layer, raising levels of harmful ultraviolet radiation.
The possibilities, based on modeling but not yet supported by solid evidence, were presented in the journal Geophysical Research Letters.
Mass extinctions have occurred in Earth's past. That much is clear, from the fossil record. But what cause them is less certain. A widespread die-off 65 million years ago, which wiped out the dinosaurs and many other species, is thought by most scientists to have been caused by an asteroid impact.
Other extinctions have been attributed to impacts, climate change, cosmic rays, exploding stars, increased volcanic activity and even global warming. Multiple events may have conspired to make life difficult in any one of the five known worst mass extinctions.
The idea that we pass through clouds of galactic debris is not new. In fact, a 2003 study found that we're traveling through a mild one right now.
How would space clouds trigger death?
"Computer models show dramatic climate change can be caused by interstellar dust accumulating in Earth's atmosphere during the solar system's immersion into a dense space cloud," said Alex Pavlov, principal author of both papers and a researcher at the University of Colorado, Boulder.
The dust layer would hover around Earth, reducing the amount of sunlight reaching the planet while letting terrestrial heat escape into space, creating a snowballing chill.
"There are indications from 600 to 800 million years ago that at least two of four glaciations were snowball glaciations," Pavlov said. "The big mystery revolves around how they are triggered."
Moderately dense space clouds, the sort that might destroy the ozone layer, are huge, Pavlov points out, and the solar system could take up to 500,000 years pass through one. Extra cosmic rays produced during such an event, owing to interactions of the interstellar dust with the Sun, would break up nitrogen molecules in Earth's atmosphere, leading to ozone destruction.
Pavlov said the work, supported by NASA, might be testable. Geologists could look for higher amounts of uranium 235 in soil layers corresponding to the time of known glaciations. Uranium 235 can't be produced naturally in the solar system.

This perspective view – the first ever produced of Mars’ north polar ice cap – shows layers of water ice and dust that lead to cliffs, which drop nearly 1.2 miles (two kilometers) down to lower terrain. Dark material in the lower, caldera-like regions may be a sign of volcanic ash.

Time to View Planet Mercury is Now

The planet Mercury is often cited as the most difficult of the five bright planets to see.
Called an "inferior planet" because its orbit is nearer to the Sun than the Earth’s, Mercury -- scarcely more than half as far from the Sun as Venus is -- always appears from our vantagepoint to be in the same general direction as the Sun and it’s usually lost in the sunlight.
Yet it’s not really hard to see. You simply must know when and where to look, and find a clear horizon.
Three times a year this rocky little world emerges into the evening twilight for a few weeks. At three other times in the year it ventures a little way into the morning sky. Yet, even at these "greatest elongations" it can’t be easily seen unless other conditions are favorable. It is important, for instance, that Mercury be positioned as nearly directly above the Sun’s place as possible, a condition that is best fulfilled just after sunset in the late winter or early spring and before sunrise in late summer or early autumn.
For those living in the Northern Hemisphere, a great window of opportunity for viewing Mercury in the evening sky has just opened up. The window will remain open through at least March 19, providing an ample number of chances to see this so-called elusive planet with your own eyes.
When and where
Currently, Mercury is visible about 45 minutes after sunset, very near to the horizon, just to the south (left) of due west. Yet, if your sky is clear and there are no tall obstructions to your view (like trees or buildings) you should have no trouble in seeing it as a very bright "star" shining with just a trace of a yellowish-orange tinge.
On the evening of March 4, Mercury will be shining at magnitude –1.1, which means that only one other object in that same evening sky will be brighter: Sirius (the brightest of all stars).
In the evenings that follow, Mercury will slowly diminish in brightness, but it will also slowly gain altitude as it gradually moves away from the vicinity of the Sun.
Be sure to cast a glance low toward the western horizon just after sunset on Friday evening, March 11. You will see a lovely, wire-thin crescent Moon, less than 1½ days after passing through New Moon phase and only 3-percent illuminated. There, less than four degrees to the right and slightly below this slender sliver will be a bright "star" shining in the twilight -- that's Mercury.
It will arrive at its greatest elongation the very next night, 18-degrees to the east of the Sun. Take note that the Moon will have moved well away from Mercury; far to its upper left. Shining this night at magnitude –0.2 (a trifle brighter than the star Arcturus, in the constellation of Boötes), Mercury sets more than 1½ hours after the Sun, making this its best evening apparition of 2005.
Just a phase
Mercury, like Venus, appears to go through phases like the Moon.
When March began, its disk was 86 percent illuminated by the Sun, giving it a distinct gibbous appearance in telescopes, which is also why it started out the month appearing so bright. By the time it arrives at its greatest elongation, it will appear roughly half-illuminated, and the amount of its surface illuminated by the Sun will continue to decrease in the days that follow.
So correspondingly, when it begins to turn back toward the Sun’s vicinity after March 12, it will fade at a rather rapid pace.
In fact, by the evening of March 19, Mercury’s brightness will have dropped to magnitude +1.5, slightly dimmer than the star Regulus in Leo; only 1/11 as bright as it was on March 4. (On the astronomers' magnitude scale, larger numbers represent dimmer objects.) In telescopes Mercury will appear as a narrowing crescent phase. Thus, in all likelihood, March 19 will be one of your last views of it.
The combination of its lowering altitude, plus its descent into a much-brighter sunset glow should finally render Mercury invisible by the final week of March. It will pass through inferior conjunction – between the Sun and Earth – on March 30.
Legends and fact
In old Roman legends, Mercury was the swift-footed messenger of the gods. The planet is well named for it is the closest planet to the Sun and the swiftest of the Sun’s family, averaging about 30 miles per second; making its yearly journey in only 88 Earth days. Interestingly, the time it takes Mercury to rotate once on its axis is 59 days, so that all parts of its surface experiences periods of intense heat and extreme cold.
Although its mean distance from the Sun is only 36 million miles, Mercury experiences by far the greatest range of temperatures: nearly 900 degrees Fahrenheit (482 Celsius) on its day side; minus 300 Fahrenheit (-184 Celsius) on its night side.
In the pre-Christian era, this planet actually had two names, as it was not realized it could alternately appear on one side of the Sun and then the other. Mercury was called Mercury when in the evening sky, but was known as Apollo when it appeared in the morning.
It is said that Pythagoras, about the fifth century B.C., pointed out that they were one and the same.

High resolution, stereoscopic images reveal a flat, "plate-like terrain" in the region of southern Elysium Planitia, near the Martian equator, that appears remarkably like fields of pack ice on Earth, they say.

Frozen Lake Found On Mars, May Have Preserved Primitive Life: Scientists

Frozen Lake Found On Mars, May Have Preserved Primitive Life: Scientists

Paris (AFP) Mar 16, 2005
Claims that life, or the potential for it, exists on Mars were boosted last Wednesday when scientists declared they saw the remains of a frozen sea on the planet's surface and speculated the ice may hold preserved organisms.
Planetary geologists led by John Murray of Britain's Open University said the evidence comes from pictures sent home by the European Space Agency's Mars Express orbiter.

High resolution, stereoscopic images reveal a flat, "plate-like terrain" in the region of southern Elysium Planitia, near the Martian equator, that appears remarkably like fields of pack ice on Earth, they say.

The "frozen lake" measures about 800 by 900 kilometers (500 by 560 miles) long and is probably about 45 metres (150 feet) deep on average, making it similar in size and depth to the North Sea.

"If our interpretation is confirmed, this is a place that might preserve evidence of primitive life, if it has ever developed on Mars," the group ventures in Nature, the British science weekly.

The apparent lake lies at the west end of Athabasca Vallis, an outflow channel leading from a trench called Cerberus Fossae.

Murray's team contends the water is the relic of an ocean created from sub-surface ice that was melted by volcanic activity and gushed to the surface along the Cerberus Fossae.

The water pooled and quickly froze, but almost immediately began to evaporate into Mars' low-pressure atmosphere.

The "lake" seen from the Mars Express pictures was possibly covered by a layer of volcanic ash and this acted like an insulative layer, slowing down the evaporative loss.

Over the past seven years, evidence has grown thanks to US and European space missions that Mars was once awash with water, one of the magical ingredients for creating life as we know it.

The seas eventually disappeared and residentual quantities of surface water remain locked up as ice at its poles.

But among the many unanswered questions, two big ones stand out: when were these oceans created? And could more water lie untapped under the surface?

A common view is that the big watery event probably happened several billion years ago, when Mars was an infant planet.

But Murray's belief is that the "lake" at southern Elysium is far more recent. It looks as if it were formed about three to seven million years ago, he contends.

Two other Mars Express analyses, also published in Nature, strengthen the theory of recent water flows on Mars.

These conclusions are based on glacial deposits formed in depresssions on the side of the volcano Hecates Tholus, and glacial activity on what is now tropical and mid-latitude regions on the Red Planet.

Evidence about water on Mars may be rapidly strengthening, but even so none has yet emerged that the planet harbours or has ever harboured life, even in the most rudimentary form.

The basis for Murray's speculation lies in the hope that beneath Mars' permanently cold crust, heat from volcanic activity may create a subterranean layer of warm water.

Over billions of years, this warm bath could help micro-organisms to evolve, even in negligible oxygen and in the absence of light, the authors argue.

Similar hardy creatures, called extremophiles, exist on Earth in deep-sea thermal vents and are common ancestors to many forms of life, they point out.

The research, which is published on Thursday, was first presented last month at a conference in the Netherlands on data sent back by Mars Express, where it created huge excitement.

If true, the frozen sea would be a prime candidate for a land mission to search for confirmation that life can exist elsewhere than on Earth.

Saturn by Cassini 2004

NASA's Mars Exploration Rover Spirit spies two dust devils in action.

Mars rover gets new lease on life

Dust devil cleans Spirit's solar arrays

SPACE.com) -- Mars scientists and engineers are elated about a dust-busting blast that has struck the Spirit rover at its Gusev crater exploration site.
Turns out that a Martian whirlwind -- dubbed a dust devil -- likely zoomed over the robot high up in the Columbia Hills. That fleeting flyby effectively cleaned Spirit's solar arrays, giving the robot a new lease on life.
Engineers report that the rover's power reading quickly shot up to almost as high as when the rover landed on Mars over a year ago.
Rover scientists suspected something was up at the Gusev site when Spirit's wheel tracks were disappearing. Onboard cameras could look down and see the tracks vanishing. Rover team members assumed that the site was experiencing a heavy dust loading in the atmosphere.
Indeed, the rover's energy quickly dropped. Seeing the robot's decreasing power level, controllers started to consider cutting back on rover Mars work.
"Gusev was alive with dust devils," explained one scientist familiar with rover operations.
But suddenly Spirit's available energy rocketed to a high level. The plus-up in power, team members believe, was due to a whirlwind passing right over the robot, removing the dust that had collected on its solar cells.
The impact of the devilish dust-off was significant.
"The noon solar output from the panels went from a 40 percent loss to just 7 percent," said rover science team member, Larry Crumpler, a research curator in volcanology and space sciences at the New Mexico Museum of Natural History and Science in Albuquerque.
Images of the panels taken later showed "beautiful dark panels," Crumpler explained. "And all the wires and edges on the [rover] deck have little dust tails. I think it might have been the Martian squeegee men. Either that or one heck of a buffeting by a dust devil," he said.
Spirit has been busy wrapping up a spectacular panorama from the vantage point of "Larry's Lookout."
Earlier this month, lead investigator for the Mars Exploration Rover mission, Steve Squyres of Cornell University, noted that Spirit's depleted power was reducing the number of hours per day available to snap photos.
Squyres couldn't gauge the chance of what he called a "miracle cleaning event" -- akin to what occurred months ago on Opportunity, its sister robot on the other side of Mars. "If it happens, I'll take it!"
"We have to assume the worst...that the solar panels are going to stay dirty and just get dirtier," Squyres told SPACE.com at the time.
As to what caused Opportunity's solar panels at Meridiani Planum to become cleaned is a puzzle, Squyres said. "Wind has to be involved at some level you figure. Frost might have helped. A frost build-up on arrays could coagulate the dust...but the fact is that we don't understand it very well. But I'll take it."

The Cassini spacecraft's two flybys of Saturn's icy moon Enceladus have revealed that the moon has a significant atmosphere.

Cassini probe detects Enceladus atmosphere

LOS ANGELES, California (Reuters) -- The space probe Cassini discovered a significant atmosphere around Saturn's moon Enceladus during two recent passes close by, the Jet Propulsion Laboratory said on Wednesday.
Scientists speculate the atmosphere comes from volcanoes, geysers or some other kind of sub-surface activity, JPL said in a statement.
"These new results from Cassini may be the first evidence of gases originating either from the surface or possibly from the interior of Enceladus," said Dr. Michele Dougherty, principal investigator for the Cassini magnetometer and professor at Imperial College in London.
Cassini made passes on February 17 and March 9 and discovered the atmosphere using a magnetometer, which discovered a magnetic field on the moon.
Because gravity is weak on Enceladus, a continuous source is needed to maintain the atmosphere, JPL said. The 310-mile-wide (500-km-wide) moon is icy and considered the most reflective object in the solar system, bouncing back about 90 percent of the sunlight that hits it.
Titan, another moon of Saturn, also has a substantial atmosphere. The European probe Huygens, which went to Saturn piggy-back on Cassini, landed on Titan in January.
Cassini-Huygens is a joint venture of NASA, the European Space Agency and the Italian Space Agency. Scientists at JPL designed, developed and built Cassini.